Method and apparatus for washing fiber pulp mixture

ABSTRACT

A method and an apparatus for washing a fiber pulp mixture, whereby water and, with it, ash and other particle-type impurities are removed from the fiber pulp mixture. The fiber pulp mixture is fed between two wires in a dewatering section so that water and ash are removed from the fiber pulp through both wires, after which the wires and the fiber pulp between them are led to run along a curved surface to remove any extra water by means of the tension of the wires and centrifugal force.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of pending PCT International Application PCT/FI99/00751, filed Sep. 14, 1999, designating inter alia the United States.

FIELD OF THE INVENTION

[0002] The invention relates to a method and an apparatus for washing fiber pulp, in which fiber pulp is fed into a dewatering section and water and, with it, ash and/or other impurities are removed through a wire running in the same direction as the fiber pulp in the dewatering section, wherein the fiber pulp being washed is fed into a dewatering section formed between two wires and the wires are arranged to run in such a way that the dewatering section converges from the feed end to the outlet end, whereby water and, with it, impurities are removed in the dewatering section through both wires, and after the dewatering section, the wires and the pulp between them are led to run along a curving surface in such a way that pressure caused by the tension of the wires and the centrifugal force produced by the curved movement cause the removal of any extra water from the pulp.

BACKGROUND OF THE INVENTION

[0003] Fiber pulp, such as recycled fiber, is washed while being processed. When fiber pulp is washed, water is removed from it in a manner that aims at removing particle-type material, such as ash, with the water while useful fibers are retained in the fiber pulp.

[0004] Apparatuses for washing fiber pulp in this manner were presented in the Tappi Conference from Jun. 15 to Jun. 19, 1998, for instance, and according to a presentation given in the conference, a good washing result is achieved by two wire-type apparatuses only. One apparatus comprises two separate large-diameter rolls around which a wire runs. In this solution, the fiber matter being washed is fed into a gap between the roll and the wire. The other apparatus comprises only one large-diameter roll and a separate wire section with associated apparatuses. In this solution, too, the fiber matter is fed into a gap between the roll and the wire. In both apparatuses, a majority of the water is removed through one wire.

[0005] A problem with both apparatuses is the small fiber matter washing capacity per meter of width of the apparatus. A further problem with both apparatuses is poor wire durability, because, in both solutions, the finer side of the wire is against the roll. In addition, neither solution provides the possibility of substantially adjusting the level of dewatering, and thereby adjusting the quantity of fines lost during washing, except by changing wires. A still further problem with both apparatuses is that the dry stuff content of the washed fiber material exiting the apparatus is relatively low and thus usually requires a separate concentration prior to further processing. Yet another problem with both apparatuses is that they require a high wire speed, which is disadvantageous with respect to both energy consumption and wire durability.

SUMMARY OF THE INVENTION

[0006] It is an object of this invention to produce a method and apparatus with which it is possible to simply and efficiently wash impurities from fiber pulp and, at the same time, to raise the consistency of the washed fiber pulp to a level sufficient for further processing.

[0007] The invention is characterized in that the pulp being washed is fed into a dewatering section between two wires, in which at least one of the wires is led to run in the dewatering section along a convex path such that the dewatering is substantially more efficient at the forward end of the dewatering section than at the tail end.

[0008] Preferably, dewatering and thus also washing occurs through both wires in the dewatering section. In a preferred embodiment, the fiber pulp coming out of the dewatering section between the wires is led together with both wires to run along a curved surface, such as the outer surface of a turning roll or a surface of a curved shoe, whereby the removal of any extra water is achieved by pressure caused by the tension of the wires and by centrifugal force produced by the curved movement. According to the invention, the fiber pulp is preferably fed to a closed dewatering chamber through which the wires ran in a manner that produces a convergent dewatering section between the wires. In accordance with preferred method and apparatus embodiments of the invention, turbulence is caused in the fiber pulp before it is fed into the dewatering section, whereby dewatering and thus also washing is performed as efficiently as possible and as far as possible along the length of the dewatering section without causing layers of fibers to be deposited on top of the wires.

[0009] In one preferred embodiment of the invention, the dewatering in the beginning of the dewatering zone is performed in such a manner that the flow rate of the fiber pulp differs from the motion speed of the wires in the beginning of the dewatering section. In this way, turbulence caused in the fiber pulp before it is fed into the dewatering section can be efficiently maintained for as long as possible. A yet another essential idea of the invention is that the dewatering section is made so as to curve towards the inside in such a manner that its cross-profile decreases more rapidly at the forward end than at the tail end. This produces a very efficient dewatering, which combined with the dewatering performed by the turning roll results in preferably achieving a high enough dry stuff content in the washed fiber material for the purpose of further processing. A further advantage of the invention is that when washing the fiber pulp, ash and other particle-type material can very efficiently be removed from the fiber pulp along with water while the fibers of desired length remain efficiently in the fiber pulp mixture. A yet further advantage of the invention is that a sufficiently high concentration of the fiber pulp is achieved after washing without additional presses or corresponding mechanical apparatuses.

[0010] In accordance with the invention, dewatering and thus also the washing result can be substantially adjusted with control valves or other control elements in the area of the dewatering chamber, which also improves the washing result and fines loss.

[0011] If a strong wash is required, dewatering is adjusted in such a manner that more water per length of fiber pulp mixture in the motion direction is removed in the forward end than in the tail end of the dewatering zone. Correspondingly, if a careful/fines-saving wash is required, dewatering is adjusted in such a manner that approximately the same amount of water or less per length of fiber pulp mixture in the motion direction is removed in the forward end than in the tail end of the dewatering zone.

[0012] The apparatus of the invention also enables additional dewatering by means of a curved surface, such as a turning roll, located after the dewatering section. No edge sealing is necessary after the actual dewatering section, because the clearance in the outlet end of the section is adjusted in such a manner that the dry stuff content of the fiber pulp mixture in the outlet end is already sufficiently high. The actual additional dewatering is done based on wire tension and centrifugal force on the curved surface.

[0013] The apparatus of the invention provides considerable advantages in comparison with other corresponding apparatuses. Dewatering and washing with the apparatus of the invention occurs in two directions, which provides a considerable capacity increase in comparison with prior apparatuses of equivalent width. The apparatus also preferably contains a separate turbulence element. With the apparatus, the flow rate of the fiber pulp mixture in the dewatering section is kept suitable in relation to the wire speed. Both factors thus contribute to effectively preventing the fiber pulp mixture from depositing onto the surfaces of the wires too early during the fiber pulp's progression along the dewatering section, thus maintaining a good washing result. The washing result and fines loss can also effectively be adjusted by controlling the dewatering in different ways in the longitudinal direction of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention will be described in greater detail in the attached drawings in which

[0015]FIG. 1 shows a schematic sectional side view of an embodiment of the apparatus of the invention,

[0016]FIGS. 2a and 2 b show schematically a structure of the apparatus of the invention after the outlet opening of the dewatering section and a detail of it,

[0017]FIG. 3 shows a schematic sectional side view of a second embodiment of the apparatus of the invention,

[0018]FIG. 4 shows a sectional side view of a third embodiment of the apparatus of the invention, and

[0019]FIG. 5 shows schematically the effect of washing on the fiber distribution of a fiber pulp implemented by the apparatus and method of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020]FIG. 1 shows a schematic sectional side view of an embodiment of an apparatus in accordance with the invention. The apparatus comprises a first wire 1 and a second wire 2, between which a dewatering section 3 is formed. The first wire 1 runs as a closed loop around guide rolls 4 a to 4 d and the second wire 2 correspondingly around a turning roll 5 and guide rolls 6 a and 6 b. The number and location of guide rolls can vary, as known to a person skilled in the art. The fiber pulp mixture to be washed is fed into a feeder chamber 7 from which it is preferably fed for instance through turbulence generator 8 to the dewatering section 3 formed between the first and second wire 1 and 2. On both sides of the dewatering section 3, against the outer surfaces of the wires 1 and 2, there are dewatering boxes 9 and 10 or the like, through which water is drained from the fiber pulp mixture in the dewatering section 3, as schematically illustrated by arrows 9 a and 10 a. The dewatering section 3 is preferably a closed chamber formed by sides and support structures between the sides, along which the wires 1 and 2 run inside the dewatering chamber. The dewatering boxes 9, 10 or the like are installed on the support structures in such a manner that the water draining through the wires runs through the support structures into the dewatering boxes or the like. Such wire support structures, which can be of perforated plate, various foil structures or the like, are generally known per se to a person skilled in the art and they or their operation, therefore, need not be described in greater detail. In this application and in the associated claims, the term “closed” as used in connection with the dewatering chamber refers specifically to a structure in which the fiber pulp and the water it contains cannot exit the dewatering section along other than the desired dewatering routes.

[0021] Because both sides of the dewatering section are closed and the area between the wires 1 and 2 and the guide rolls 4 a and 6 b are sealed so as to prevent the fiber pulp mixture from running in any other direction, the fiber pulp mixture has to flow from the dewatering section 3 onward in the same direction as the wires 1 and 2. Simultaneously, water is removed from it along the entire way. Dewatering occurs in the dewatering section for instance in such a manner that preferably more water per length of fiber pulp mixture in the motion direction is removed at the forward end of the dewatering section 3, i.e., the incoming end of the fiber pulp, than at the tail end of the dewatering section 3. When turbulence is caused in the fiber pulp mixture prior to feeding the mixture into the dewatering section, the turbulence can be maintained by adjusting the dewatering based on the speed ratio or difference between the mixture and the wires, such that the fibers stay suitably mixed without any significant deposition on the surfaces of the wires 1 and 2.

[0022] When the fiber pulp mixture arrives at the turning roll 5, additional dewatering is performed by means of the centrifugal force and the tension of the wires. The additional water thus removed flows up through the wire 1 to a dewatering tray 11, resulting in a fiber pulp with a suitable dry stuff content. Since the concentration of the fiber pulp mixture when coming out of the dewatering section 3 is rather high, the fiber pulp mixture stays between the wires 1 and 2 without gushing out from the side. Thus, this apparatus does not need any separate sealing after the dewatering section. Making the fiber pulp follow the lower wire 2 requires some kind of a force acting in that direction. This can be advantageously achieved by using a smooth roll as the turning roll 5. Thus, a negative pressure forms on the left side of the turning roll 5 in the space between the wire 2 and the turning roll 5 where the wire 2 detaches from the roll. This space is marked with reference number 12 in the figure. This causes the low pressure to draw the fiber pulp against the wire 2 and thus the fiber pulp is detached from the top wire 1.

[0023] By suitably designing the upper and lower surfaces of the dewatering section, the dewatering distribution along the length of the dewatering section can be altered in many ways. For instance, if in the forward end of the dewatering section, water is drained from the fiber pulp mixture faster than the speed with which the cross-sectional area of the dewatering section decreases, the flow speed of the fiber pulp decreases in this area. Another consequence is that closer to the outlet end of the dewatering section, water is drained slower than the speed with which the cross-sectional area of the dewatering section decreases, which results in a fiber pulp mixture speed at the outlet opening which equals the motion speed of the wires. Conversely, if, in the forward end of the dewatering section, less water is drained than should in relation to the decreasing of the cross-sectional area of the dewatering section, the speed of the fiber pulp mixture becomes higher than the motion speed of the wires, and in this case dewatering closer to the outlet opening of the dewatering section must be greater in relation to the decrease in the cross-sectional area of the dewatering section so that the speed of the fiber pulp mixture at the outlet opening is the same as the motion speed of the wires 1 and 2. Both alternatives can be achieved for instance by constricting the dewatering channels and thus adjusting the outflow, or by various other generally known measures. However, the result is that a flow speed of fiber pulp mixture differing from the motion speed of wires maintains/causes turbulence and thus aids the operation of the apparatus in certain conditions.

[0024] In a fiber pulp wash in which ash and other material in the fiber pulp mixture need to be removed as thoroughly as possible, water can be fed into the fiber pulp mixture before the turning roll 5 from nozzles 13 located below the wire 2. In this case, the water flowing through the fiber pulp mixture transports these particle-type solids to the other side of the fiber pulp mixture and on through the wire 1, after which both water and particles are hurled into the dewatering tray 11.

[0025]FIG. 2a shows schematically a structure of the apparatus of the invention immediately after the outlet opening of the dewatering section before the wires and the fiber pulp arrive at the turning roll 5. The figure shows how the dewatering zone is everywhere surrounded by either the higher or the lower support structure, such as cover part 3 a or 3 b or sides 3 c, and the wires 1 and 2 correspondingly press against the higher and the lower support structures. The fiber pulp mixture fills the space between the wires 1 and 2 and the dewatering section from the wires towards the edges. The first wire 1 higher in the figure is preferably slightly narrower than the lower second wire 2. With this arrangement, when wires 1 and 2 and the fiber pulp mixture come out of the dewatering section 3, water jets 14 on the side of the higher wire can wash away the portion of the fiber pulp mixture which remains outside the wire 1, whereby the edges of the second wire 2 are cleaned of the fiber pulp mixture and the mixture thus cannot stick to other parts of the apparatus and thus unnecessarily dirty the apparatus. This is described more closely in FIG. 2b illustrating this detail. A dashed line surrounds the portion of the fiber pulp mixture between the wires 1 and 2, which due to the water jets 14 moves towards the direction shown by arrow A and thus leaves the edge of the wire 2 clean.

[0026]FIG. 3 shows a sectional side view of a second embodiment corresponding to the embodiment of the apparatus in FIG. 1, except that a separate dewatering shoe 15 has been added to it between the dewatering section and the turning roll 5, along which shoe the wires 1 and 2 run to remove water from the fiber pulp mixture. The dewatering shoe 15 advantageously has a curved surface, preferably so that its curve is smaller than the curve of the turning roll 5 following it, or of any other corresponding curved surface. In this way, dry stuff content can be raised even further without complex and expensive presses or other corresponding apparatuses. Further, the figure shows pressure pulse elements, in this case separate strips 16, located in the dewatering section at the second wire 2. These strips produce an alternating pressure pulse through the wire, in which one pulse part pushes the fiber pulp on the surface of the wire 2 away from the wire and a counter-directional pressure pulse causes suction towards the wire, whereby the fiber matter layer accumulating on the surface of the wires is broken up, thus considerably reducing its tendency to prevent or interfere with dewatering. Additionally, turbulence can be maintained in the dewatering section thus improving dewatering without flocking and layering on the wire surface. Instead of the strips 16, various rotating pressure pulse elements or other solutions known in the art can be used to produce pressure pulses on one or both sides of the dewatering section.

[0027]FIG. 4 shows a third embodiment of the apparatus of the invention, in which the turning roll 5 is replaced by a curved shoe 17 around which the wires and the fiber pulp mixture turn. The shoe can have a surface with strips, or be implemented in any other manner known per se, whereby water can also be sucked through the wire 2, or strips can be used to produce suction to improve dewatering.

[0028]FIG. 5 shows schematically the effect of washing on the fiber distribution of a fiber pulp implemented by the apparatus and method of the invention. In the test in question, the distribution of fibers of different lengths is measured by a measuring method known per se and the figure illustrates the proportion of the fibers of different lengths in the fiber pulp used in the test before and after washing. In this figure, I represents the proportion of the longest fibers and V the proportion of the fines and ash in the pulp. As shown in the figure, a significant amount of parts in the V group, i.e., ash and fines, are removed during washing, and as a result, their relative proportion in the washed fiber pulp is substantially reduced. At the same time, the proportion of other fiber contents in the washed pulp is correspondingly higher. The figure shows clearly that washing in accordance with the invention removes the undesirable components of the pulp, and the desirable fibers tend to remain in the pulp.

[0029] Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. 

What is claimed is:
 1. A method for washing a fiber pulp mixture, comprising the steps of: feeding the fiber pulp mixture into a feed end of a dewatering section formed between two moving wires arranged such that the dewatering section converges from the feed end to an outlet end thereof; removing water, and consequently impurities, from the fiber pulp mixture through each of the two wires in the dewatering section; and leading the wires with the fiber pulp mixture therebetween along a curved surface downstream of the dewatering section so as to remove additional water from the fiber pulp mixture by centrifugal force and pressure of the wires; wherein at least one of the wires in the dewatering section travels along a convex path arranged so as to make the dewatering substantially more efficient at a forward end of the dewatering section than at a tail end thereof.
 2. The method of claim 1, wherein the wires in the dewatering section travel through a dewatering chamber having closed sides arranged closely adjacent opposite edges of the wires.
 3. The method of claim 1, wherein turbulence is caused in the fiber pulp mixture before feeding the fiber pulp mixture into the dewatering section.
 4. The method of claim 3, wherein the turbulence is generated by a separate turbulence generation element.
 5. The method of claim 1, wherein the step of leading the wires along a curved surface comprises leading the wires about a surface of a turning roll.
 6. The method of claim 1, wherein the step of leading the wires along a curved surface comprises leading the wires about a surface of a curved shoe.
 7. The method of claim 1, further comprising the step of feeding water between the curved surface and the wire thereon to enhance washing of the fiber pulp mixture.
 8. The method of claim 1, further comprising the step of causing pressure pulses in the fiber pulp mixture on at least one side along a length of the dewatering section.
 9. The method of claim 8, wherein the pressure pulses are caused by a separate pressure pulse element located in the dewatering section proximate an outer surface of one of the wires.
 10. An apparatus for washing a fiber pulp mixture, comprising: a dewatering section comprising a pair of moving wires arranged in opposition to each other and converging toward each other from a forward end to a tail end of the dewatering section, the fiber pulp mixture being fed into the forward end of the dewatering section and water along with impurities being removed from the fiber pulp mixture through each of the wires in the dewatering section; and a curved surface located downstream of the dewatering section, the wires with the fiber pulp mixture therebetween being arranged to travel along the curved surface such that additional water is removed from the fiber pulp mixture by centrifugal force and pressure of the wires; wherein at least one of the wires in the dewatering section travels along a convex path.
 11. The apparatus of claim 10, wherein the dewatering section further comprises a dewatering chamber through which the wires pass, the dewatering chamber having closed sides located adjacent opposite edges of the wires.
 12. The apparatus of claim 10, further comprising a turbulence element for generating turbulence in the fiber pulp mixture being fed into the dewatering section.
 13. The apparatus of claim 10, wherein the curved surface comprises a surface of a turning roll.
 14. The apparatus of claim 10, wherein the curved surface comprises a surface of a curved shoe.
 15. The apparatus of claim 10, further comprising elements for producing pressure pulses in the fiber pulp mixture in the dewatering section.
 16. The apparatus of claim 10, wherein the wires are of different widths such that fiber pulp can be washed off marginal edge portions of a wider one of the wires downstream of the dewatering section.
 17. The apparatus of claim 10, wherein the wires pass between suction boxes in the dewatering section and at least one of the suction boxes presents a curved surface so as to produce said convex path for the wire.
 18. An apparatus for washing a fiber pulp mixture, comprising: a dewatering section comprising a pair of wires traveling along a running direction and arranged in opposition to each other and converging toward each other from a forward end to a tail end of the dewatering section, the fiber pulp mixture being fed into the forward end of the dewatering section and water along with impurities being removed from the fiber pulp mixture through each of the wires in the dewatering section, wherein at least one of the wires in the dewatering section travels along a convex path and the wires are arranged such that a cross-sectional area of the dewatering section decreases at a greater rate in the running direction at the forward end of the dewatering section than at the tail end thereof.
 19. The apparatus of claim 18, wherein the dewatering section further comprises a dewatering chamber through which the wires pass, the dewatering chamber having closed sides located adjacent opposite edges of the wires.
 20. The apparatus of claim 18, further comprising a turbulence element for generating turbulence in the fiber pulp mixture being fed into the dewatering section.
 21. The apparatus of claim 18, further comprising elements for producing pressure pulses in the fiber pulp mixture in the dewatering section.
 22. The apparatus of claim 18, wherein the wires are of different widths such that fiber pulp can be washed off marginal edge portions of a wider one of the wires downstream of the dewatering section. 